Conventionally, the semiconductor devices used in power modules and the like use circuit boards in which a conductive layer of Cu or Al, and a heat radiating plate are formed on the front and rear surfaces of a ceramic substrate consisting of alumina, beryllia, silicon nitride or aluminum nitride or the like. This type of circuit board enables a high level of insulation to be obtained in a more stable manner than composite substrates formed from a resin substrate and a metal substrate, or resin substrates.
In those cases where the conductive layer and the heat radiating plate are Cu, the generation of thermal stress caused by the differing levels of thermal expansion of the ceramic substrate and the solder is unavoidable, and as a result, the long term reliability tends to be unsatisfactory. In contrast, in those cases where the conductive layer and the heat radiating plate are Al, although the thermal conductivity and electrical conductivity are inferior to those of Cu, the Al can readily undergo plastic deformation if exposed to thermal stress, enabling the thermal stress to be alleviated, and as a result, the reliability is superior.
Japanese Unexamined Patent Application, First Publication No. 2001-53199 and Japanese Unexamined Patent Application, First Publication No. Hei 8-335652 disclose related technology.
In order to bond rolled Al plates to both surfaces of an insulating ceramic, bonding must be conducted at a high temperature of at least 500° C., using a brazing material. In such cases, following the brazing process, warping can develop when the structure is returned to normal temperatures, and this warping can cause problems in the circuit board production process and the assembly process used for producing power modules.
As shown in FIG. 3A, it is thought that the cause of this warping is the Al crystals within the conductive layer growing overly large during the brazing process. If the crystal grain diameter becomes overly large, then anisotropy develops in the mechanical characteristics of the conductive layers formed on both surfaces of the insulating ceramic substrate, and the resulting unbalanced stresses cause warping. In those cases where conductive layers are bonded to both the front and rear surfaces of an insulating ceramic substrate, the two conductive layers are preferably of the same thickness in order to ensure better stress balance.
The grain diameter of Al crystals can be controlled by increasing the quantity of added elements, although as the quantity of added elements increases, the ability of the Al to alleviate stress is reduced. As a result, if the 0.2% proof stress value or the work hardening coefficient of the Al exceed certain reference values, then when, for example, −40° C. to 125° C. temperature cycle testing is conducted, the ceramic substrate may develop cracking.
The present invention takes the above circumstances into consideration, with an object of reducing circuit board warping, and preventing ceramic substrate cracking.